why is the diameter different on these two Bezier curves using the same Bezier circle as a bevel object?
If you enter edit mode and select the vertices of the curves you can see it in the n panel that the radius of the vetices of the thicker one is set to 2.5 while the other one has vertices with radius of 1.


If the profile needs to be a simple circle you do not need to use another curve as Bevel shape at all. You could also just set the fill type to Full in the curve properties and then set a desired Bevel Depth. You will need to increase the resolution for it to be round as well:



It might be a few clicks less to set it up this way.